1. Field of the Invention
This invention relates to an improved process for the conversion of natural gas into valuable liquid hydrocarbon products by subjecting the natural gas to partial oxidation or autothermal reforming to produce synthesis gas and converting the synthesis gas into valuable products using a Fischer-Tropsch (FT) reactor.
2. Chemistry
The partial oxidation (POX) reaction can be expressed as:
CHz+xc2xdO2xe2x86x92z/2H2+CO 
where z is the H:C ratio of the hydrocarbon feedstock. The water gas shift (WGS) reaction also takes place: 
The Fischer-Tropsch (FT) synthesis reaction is expressed by the following stoichiometric relation:
2n H2+nCOxe2x86x92CnH2n+nH2O 
The aliphatic hydrocarbons produced by the Fischer-Tropsch reaction have an H:C atom ratio of 2.0 or greater.
Catalysts such as iron-based also catalyze the water gas shift (WGS) reaction: 
If all of the water produced in the FT reaction were reacted with CO in the WGS reaction, then the overall consumption of hydrogen would be one-half of the consumption of carbon monoxide. If none of the water were reacted in the WGS reaction (no WGS activity) then the consumption of hydrogen would be twice the consumption of carbon monoxide.
3. Description of the Previously Published Art
For a natural gas feedstock which contains no or little carbon dioxide, Benham et al. (U.S. Pat. Nos. 5,620,670 and 5,621,155) teach that carbon dioxide recycle (including carbon dioxide produced in the synthesis step) back to the synthesis gas producing step (either partial oxidation, autothermal reforming or steam reforming) decreases the excessively high H2:CO ratio of the synthesis gas and increases the yield of the Fischer-Tropsch (FT) hydrocarbons and the attendant carbon conversion-efficiency. The aforementioned patents also teach that recycling both tail gas and carbon dioxide back to the synthesis gas producing step can be used to effect an increase in hydrocarbon yields.
Yarrington et al (U.S. Pat. No. 5,023,276) describe a gas to liquids system wherein synthesis gas is produced using autothermal reforming of natural gas with carbon dioxide recycled from the outlet of the autothermal reformer back to the inlet of the autothermal reformer. Means are also provided for recycling tail gas from the Fischer-Tropsch reactor back to the autothermal reformer inlet.
Agee (U.S. Pat. Nos. 4,833,170 and 4,973,453) describes a gas to liquids system which uses autothermal reforming of natural gas with air as the oxidizing gas and a cobalt-based Fischer-Tropsch reactor. Means are provided for combusting tail gas and light hydrocarbons and for recovering carbon dioxide from the flue gases. Some of the carbon dioxide is recycled back to the inlet of the autothermal reformer to increase the yield of liquid hydrocarbon product. The amount of carbon dioxide in the feed gas to the autothermal reformer in the example given is about 4.3 volume percent.
It is an object of this invention to provide a Fischer-Tropsch process with a first stage gasifier which uses natural gas as the feedstock.
It is a further object of this invention to increase the hydrocarbon yields from a POX/FT or an ATR/FT system.
It is a further object of this invention to increase the hydrocarbon yields from a POX/FT or an ATR/FT system by separating hydrogen from the tail gas and recycling the hydrogen back to the FT reactor or to the POX or ATR reactor or to both.
It is a further object of this invention to increase the H2:CO ratio of the synthesis gas fed to the FT reactor by using hydrogen recycle.
It is a further object of this invention to increase the H2:CO ratio in a POX/FT or an ATR/FT system so as to increase catalyst stability and life.
It is a further object of this invention to recycle hydrogen back to the POX or ATR reactor in a POX/FT or an ATR/FT system so that less steam and oxygen are required in the POX or ATR reactor.
These and further objects of the invention will become apparent as the description of the invention proceeds.
Increased hydrocarbon product yields and reduced oxygen consumption improvements are obtained in a Fischer-Tropsch (FT) gas-to-liquids conversion apparatus by the use of additional apparatus to selectively recycle hydrogen, carbon dioxide and/or tail gas from the FT reactor. The apparatus has a first unit which is a synthesis gas production reactor for producing synthesis gas from a natural gas feedstock. Examples of such reactors are a partial oxidation (POX) reactor or an autothermal reactor (ATR). The second unit is a synthesis gas conversion apparatus which is the FT reactor. The FT reactor can have a catalyst exhibiting either a low water gas shift (WGS) activity such as cobalt or a high water gas shift (WGS) activity such as iron. The improved results are obtained by using a hydrogen gas separating and recycling system for separating the hydrogen from the tail gas exiting the FT reactor and recycling at least a portion of the separated hydrogen back to the inlet of the FT reactor or the synthesis gas production reactor. In addition, depending on the nature of the oxidizing gas used in the synthesis gas production reactor, the nature of the catalyst in the FT reactor, and whether a POX or ATR unit is employed, (1) a tail gas recycling system may be employed for recycling at least a portion of the remaining tail gas, either before or after the hydrogen has been removed, to the inlet of the synthesis gas production reactor or (2) a carbon dioxide gas separating and recycling system may be employed for separating the carbon dioxide from the tail gas exiting from the FT reactor and recycling at least a portion of the carbon dioxide to the inlet of the synthesis gas production reactor.
In the case where oxygen is used as the oxidizing gas and the FT reactor has a catalyst with a high water gas shift (WGS) activity such as an iron catalyst, then improved results can be obtained by recycling the separated hydrogen to either the FT unit or the POX or ATR reactor and recycling at least a portion of the tail gas, either before or after the hydrogen is removed, back to the POX or ATR reactor. In the case where the hydrogen is removed from the tail gas and recycled back to the POX or ATR unit and the remaining tail gas without the hydrogen is recycled back to the POX or ATR unit, it is preferred to recycle 85% to 100% of the hydrogen and 70% to 80% of the tail gas to the POX or 80% to 90% of the tail gas to the ATR. When the hydrogen is recycled to the FT unit and the remaining tail gas without the hydrogen in recycled back to the POX or ATR unit, it is preferred to recycle 85% to 100% of the hydrogen and 70% to 80% of the tail gas to the POX or 80% to 90% of the tail gas to the ATR. When a portion of the tail gas is recycled directly back to the POX or ATR unit and the hydrogen is removed from the remaining tail gas for recycle to the POX or ATR unit, then it is preferred to recycle 85% to 100% of the hydrogen to the POX or ATR and 70% to 80% of the tail gas to the POX or 80% to 90% of the tail gas to the ATR. Finally, when a portion of the tail gas is recycled directly back to the POX or ATR unit and the hydrogen is removed from the remaining tail gas for recycle to the FT unit, then it is preferred to recycle 85% to 100% of the hydrogen to the POX or ATR and 70% to 80% of the tail gas to the POX or 80% to 90% of the tail gas to the ATR.
In the case where oxygen is used as the oxidizing gas and the FT reactor has a catalyst with low water gas shift (WGS) activity such as a cobalt catalyst, then improved results can be obtained by recycling the separated hydrogen to either the FT unit or the POX or ATR reactor and recycling at least a portion of the tail gas, either before or after the hydrogen is removed, back to the POX or ATR reactor. In the case where the hydrogen is removed from the tail gas and recycled back to the POX or ATR unit and the remaining tail gas without the hydrogen is recycled back to the POX or ATR unit, it is preferred to recycle 85% to 100% of the hydrogen and 35% to 55% of the tail gas to the POX or 60% to 80% of the tail gas to the ATR. When the hydrogen is recycled to the FT unit and the remaining tail gas without the hydrogen in recycled back to the POX or ATR unit, it is preferred to recycle 85% to 100% of the hydrogen and 35% to 55% of the tail gas to the POX or 60% to 80% of the tail gas to the ATR. When a portion of the tail gas is recycled directly back to the POX or ATR unit and the hydrogen is removed from the remaining tail gas for recycle to the POX or ATR unit, then it is preferred to recycle 85% to 100% of the hydrogen to the POX or ATR and 35% to 55% of the tail gas to the POX or 60% to 80% of the tail gas to the ATR. Finally, when a portion of the tail gas is recycled directly back to the POX or ATR unit and the hydrogen is removed from the remaining tail gas for recycle to the FT unit, then it is preferred to recycle 85% to 100% of the hydrogen to the POX or ATR and 35% to 55% of the tail gas to the POX or 60% to 80% of the tail gas to the ATR.
In the case where air is used as the oxidizing gas in a POX reactor, and the FT reactor has a catalyst with low WGS activity, then improved results can be obtained by recycling the separated hydrogen to either the FT unit or the POX reactor. It is preferred to recycle back 80 to 100% of the hydrogen from the FT reactor. The carbon dioxide can optionally be recycled to the POX reactor in an amount of 80% to 100%. In the case where air is used as the oxidizing gas in an ATR reactor, and the FT reactor has a catalyst with low WGS activity, then improved results can be obtained by recycling the separated hydrogen to either the FT unit or the ATR reactor and recycling at least a portion of the carbon dioxide back to the ATR reactor. It is preferred to recycle back 85 to 100% of the hydrogen from the FT reactor and 80-100% of the carbon dioxide back to the ATR.
In the case where air is used as the oxidizing gas in an POX reactor, and the FT reactor has a catalyst with a high WGS activity, then improved results can be obtained by recycling the separated hydrogen to either the FT unit or the POX reactor and recycling at least a portion of the carbon dioxide back to the POX reactor. It is preferred to recycle back 85% to 100% of the hydrogen from the FT reactor and 55-75% of the carbon dioxide back to the POX. In the case where air is used as the oxidizing gas in an ATR reactor, and the FT reactor has a catalyst with high WGS activity, then improved results can be obtained by recycling the separated hydrogen to either the FT unit or the ATR reactor and recycling at least a portion of the carbon dioxide back to the ATR reactor. It is preferred to recycle back 85 to 100% of the hydrogen from the FT reactor and 80-95% of the carbon dioxide back to the ATR.